Scatterometry is currently used in the semiconductor industry to measure thickness and optical properties of thin films as well as the critical dimension (CD) and profile shape of periodic structures on a semiconductor wafer. Scatterometry is also perceived as a possible future technology for overlay error metrology for the 32 nm technology node and beyond. In principle, scatterometry has clear advantages over the current imaging technology of overlay metrology. Scatterometry is capable of measuring device-size structures which cannot be resolved by imaging. Scatterometry is also thought to be more robust to process variations and asymmetry in the profile of the measured structure. Apparatus and methods for measuring overlay error using scatterometry are described in U.S. patent application Ser. No. 10/729,838, entitled “Apparatus and Methods for Detecting Overlay Errors Using Scatterometry” and filed on Dec. 5, 2003, which is herein incorporated by reference.
The most prevailing methods for measuring profile asymmetry are critical dimension scanning electron microscopy (CD-SEM) and scatterometry. The CD-SEM approach is very slow and expensive. The current implementation of scatterometry CD metrology, which is also suitable for monitoring properties of the profile (including profile asymmetry), relies on detailed modeling and is therefore also rather slow. In addition, it is very difficult to accurately model complicated profiles, such as two gratings (one on top of the other) separated by a layered possibly non-flat film. Such structures reflect the structure of devices more accurately than the single-layer grating used for scatterometry CD metrology.
Scatterometry measurements can be carried out in unpolarized reflectometry mode or in ellipsometry mode, as described in U.S. patent application Ser. No. 10/729,838. In both cases, overlay information is extracted without any need for modeling, which makes scatterometry overlay metrology very efficient (relative to scatterometry CD metrology). The ellipsometry measurement contains detailed information about the polarization of light, which is absent from the unpolarized reflectometry measurement. This additional information contained in the ellipsometry signal can result in a higher sensitivity to overlay errors and, thus, gives this technique a significant advantage over unpolarized reflectometry. The question, however, is how to extract the overlay information encoded in the ellipsometry signal. Previous approaches to using scatterometry to measure overlay error rely on a particular symmetry property of the ellipsometry signal; i.e. that the signal is an even function of the overlay. In our terminology, ellipsometry signals which posses this property are symmetric signals. Not all ellipsometry signals are symmetric. For the simple case of a one-dimensional (1D) grating target with the incident beam of light perpendicular to the grating lines (hereinafter referred to as the “Azimuth-0” case), all ellipsometry signals are symmetric. For other cases of general relative orientation between the incident light and the grating lines (conical diffraction), some ellipsometry signals are asymmetric. For these cases a methodology has to be defined which allows measurements of signals of a well defined symmetry.
Accordingly, what is desired is an ellipsometry or more generally scatterometry method for extracting overlay information for any relative orientation of the incident light beam with respect to a 1D grating target (conical diffraction), as well as for targets comprising general 2D gratings.